With prices always under pressure and stricter environmental
legislation being implemented, refineries are increasingly
forced to look for new ways of processing heavy residues.
Rising price differences between light low-sulfur and heavy
high-sulfur crude oils produce greater amounts of residues at
refineries. Stricter environmental laws will further
restrict the incineration of heavy residues at power plants due
to the emissions, and limit the sulfur
content of marine fuel oils (MFOs) to 0.1% from 2015 onward.
The current sulfur content is 1.5%. The sale via MFO is
presently the last possible outlet for many refineries to get
rid of heavy residues. Alternatively, other conversion systems,
such as visbreakers and cokers provide for a deeper processing
of heavy residues and lead to substantial investments. These
technologies also produce smaller amounts of residues that are
in need of disposal in an environmentally sound manner.

The enrichment of highly condensed aromatic hydrocarbons in
the form of aromatics, resins and asphaltenes, not to mention
sulfur and nitrogen compounds and metal contaminantssuch
as vanadium (V), nickel (Ni) and iron (Fe)takes place in
the heavy residues. From an ecological point of view, the heavy
residues constitute a significant problem. Thus, refineries are
increasingly interested in highly efficient residue
technologies, allowing heavy residues to be processed up to
100%.

Sustainable combination

Though solvent deasphalting (SDA) and bitumen production are
well-known and widely used technologies, combining these two
processes makes it possible to approach the residues issue in a
sustainable way. Adjusting the SDA process to bitumen-capable
feed production provides an opportunity to convert heavy refinery residues (particularly
vacuum residues) into products for the marketplace.

In SDA, heavy residues are split by extraction, using
solvents (such as propane, butane or pentane) at under-critical
or overcritical conditions, into deasphalted oil (DAO) and a
pitch (asphalt). At the fuel refinery, the DAO can be further
processed in a fluid catalytic cracking (FCC) plant or a
hydrocracker to become valuable gasoline, diesel and fuel oil
components. At the lube oil refinery, it can be used as bright
stock for lube oil production.

The bitumen-capable pitch is mixed with flux
componentslike vacuum gasoil (VGO), slop wax and
extractsand processed at a bitumen processing plant,
where it undergoes a specific blowing process to become
high-quality bitumen products (Fig. 1). The
weatherproof inclusion of accompanying components taking place
in the bitumen production leads to an environmentally friendly solution
for the heavy residue pollutants.

The DAO quality depends on the feed specification for the
downstream process. In the catalytic processes, the
requirements are particularly determined by the catalysts
employed. Table 1 lists the typical feed
specifications for certain follow-up processes.

The SDA process can be tailored to the necessary
specifications by adjusting certain process variables. It can
be conducted both under-critically and overcritically. By
increasing the pressure, especially in an overcritical mode,
substantial yields in the 60% to 70% range can be obtained in
relation to the feedstock. However, there is a
disadvantage: the DAO quality deteriorates substantially and
the required quality criteria for the follow-up process can
often no longer be maintained. The overcritical SDA mode is
often used to separate resins, and that is no solution when it
comes to bitumen production.

The DAO yields may also be increased by selecting specific
solvents and mixtures for different components. For the SDA
process and the lube oil production propane is usually used as
a solvent. Propane ensures the quality criteria as listed in
Table 1 for the lube oil plants. Meanwhile,
adding a small amount of n-butane to propane gives a
considerably higher DAO yield. The higher the yield, the more
the quality decreases.

The DAO yield may also be increased up to a certain extent
by increasing the solvent amount. Looking at the necessary
downstream solvent separation from the extract and the
raffinate, the solvent amount used should be minimal.

For the residue technology within an SDA and a
bitumen unit, not only must the DAO have the required quality,
but the pitch should be a bitumen-capable feed. Whether a pitch
is bitumen-capable or not can be estimated based on a feed
screening. Except for the main bitumen parameters, chemical
composition (like paraffin and asphaltene content) and thermal
stability need to be investigated. Final evaluation of bitumen
quality is possible after the complete test program is
completed. This includes all process steps required for bitumen
production. The basic pitch tests, like penetration, softening
point, penetration index and aging resistance will reveal the
feeds bitumen capability.

Table 2 lists the required pitch criteria
from the SDA for the production of bitumen type 50/70. In
addition, the pitch of two refinery residuesa vacuum
residue (VR) and a visbreaker tar (VT) from the SDAare
compared with the required criteria.

Analysis

The VR pitch shows that the penetration index (1.96)
is far below the 1.5 value. Further, the remaining
penetration after testing at 163°C is not reached. This
means that the VR pitch is not a bitumen-capable feed and can
not be used.

The criteria comparison for VT pitch reveals substantial
deviations in four major points. The penetration index
(1.91) is far too low, as is that of the VR, and the
thermal stability is too low. In addition, the kinematic
viscosity, at 135°C, is too low. Especially problematic and
typical for a visbreaker residue is the extremely high negative
change in mass (1.67) and the insufficient remaining
penetration (24) of the VT pitch. The last two criteria show
that the VT pitch is not bitumen-capable.

The VR pitch can be further optimized in the SDA plant,
meaning the pitch stability can be further improved by yield
increase. It is confirmed that the heavy residues from
visbreakers provide no bitumen-capable feed, even if an SDA is
incorporated.

The lowering of the viscosity and the instability of the
visbreaker products due to thermal cracking usually preclude
the production of high-quality bitumen products.

At the pilot plant for this concept, blowing tests will
always be necessary for a final assessment of the feedstocks bitumen capability.
The tests also contribute to decision-making as to whether a
bitumen plant makes sense. The SDA process is also optimized so
that both a high-quality DAO and a bitumen-capable pitch can be
produced. Finally, the decision to incorporate this combined
residue technology depends on the compliance
with required criteria both for the DAO and the pitch.

Pilot plants do exist for SDA tests in which the quality
parameters for various heavy residues (Fig. 2)
are assessed. Tests can be carried out for under-critical or
overcritical conditions where a range of solvents and solvent
mixtures are used at different temperatures and solvent
quantities. Special analyzers for saturated aromatics, resins
and asphaltenes (SARA) feed analysis (Fig. 3)
and end products have been set up. In connection with the
entire process parameter determination and other analytical
data, the process can be simulated and result in the design of
an SDA plant. The SARA analysis permits an optimal structure
analysis of the mixtures for the theoretical calculation of the
thermodynamic process. The data and analysis for a scale-up are
supported by the results from, and experience with, a fully
equipped reference plant (Fig. 4) for propane
deasphalting (PDA). The plant supplies a high-quality DAO as
bright stock for the base oil production.

The pitch from the SDA plant can be tested in a pilot plant
like the one in Fig. 5.

Fig.
5. Bitumen pilot plant in Vienna,
Austria.

Moving forward

The close coordination of the tests is the basis for
building a complete residue technology for a refinery. The combination of these
two technologies is also the most cost-effective method of
residue processing and gives the customer a maximum economic
advantage. The use of this technology requires, however, that
both the DAO quality for the follow-up process and the
pitchs bitumen capability are guaranteed to produce
high-quality bitumen. HP

Have your say

All comments are subject to editorial review.
All fields are compulsory.